How to add to my 2-chanl int. amp for home theater

I have a good two-channel integrated amp I use for music. I'd like to move to home theater inexpensively, that is without replacing my entire system. How can I add to what I have to get surround sound processing and amplification for the other channels? Ideally, I'd like a surround sound receiver that has line-level outs for the front right and left channels, so I can use my integrated amp. for those, and use the receiver for the other channels. Unfortunately, I haven't been able to find one that has them. The closest is receivers that have pre-amp outs. But that would bypass the preamp stage of my integrated amp. even for music, and an inexpensive surround receiver won't have as good a preamp as my integrated amp. I'd like to spend $500 or less but will consider more expensive solutions.